The invention relates generally to a method for providing a protective coating, similar to natural green-colored patina, on rolled strips or sheets made of copper such as are used for roof coverings and facade facings. More particularly, the method relates to a method that can be practice on an industrial scale.
Under normal atmospheric influences, copper will form a green to turquoise-colored coating or protective layer (the so-called patina) only after a relative long period of time, and even then only under certain conditions. The speed of formation of this natural patina is dependent, for example, on the following parameters:
Composition of the atmosphere (humidity, contaminant content); PA0 Stress caused by solid particles (sand, dust); PA0 Ambient temperature; PA0 Incline of the surface of the copper components; and PA0 Position of the copper surface relative to the direction of the wind and weather to which it is exposed. PA0 In ocean air: after about 6 years; PA0 In a big city industrial atmosphere: after about 5 to 8 years; and PA0 In a normal city atmosphere: after about 8 to 12 years. PA0 An extremely uniform pre-treatment of all strips and sheets of copper is achieved. This is an important prerequisite for providing artificial green patination of a uniform color; PA0 Variable production lengths are possible without creating scrap; PA0 No color changes occur at the ends of the strips or sheets; PA0 No failures of the grinding belt occur, due to ends of strips or sheets projecting upwards; and PA0 Targeted control, for example of the grinding belt, is possible by regulation of the strip center or strip edges.
In "Chemische Farbung von Kupfer und Kupferlegierungen" ("Chemical Coloration of Copper and Copper Alloys" published by the German Copper Institute), the following times are given for the formation of a natural patina under various atmospheric influences:
The patina effectively protects the copper component against the further corrosive effect of weathering, so that the erosion rate of the copper becomes negligible for the highest basicity of the patina. Consequently, researchers became interested in imitating this protective layer at an early point in time. For example, the artificial green patination of copper objects, such as of statues and vessels by means of the stippling technique, has been known for a long time.
A number of patination solutions capable of inducing the desired reaction have also been applied to copper elements having broad surface areas, such as roof coverings. These larger elements have generally been treated in several working steps using spraying or brushing techniques. It is also known to pre-treat individual sheets of copper with a patination solution prior to their use as roof coverings.
The disadvantage of these methods is that either they require an excessive amount of time-consuming manual labor, or they are applicable only for limited areas, and therefore are not economical for industrial-scale production.
Finally, a method is also known in which patina-colored plastic layers are applied to copper strips intended for roof coverings and facade facings. A disadvantage of this method is that the layers applied can easily splinter off. Because the natural self-healing mechanism of the natural patina is absent, the underlying copper surface begins to degrade as soon as the layer begins to splinter off. Furthermore, the processing techniques required by this method, such as welding or edging, can only be carried out within certain restrictions.
This invention is directed to the problem of further developing a method for providing copper surfaces with a green, securely adhering cover layer that is very similar to natural patina, and which can be produced on the surfaces of semi-finished products made of copper on an industrial scale.